This is one of my hopes for GC3, that diplomacy takes a step forward in terms of depth and mechanics. Diplomacy in these Civ type games is always so basic and seems to be thrown in just because you need some way to communicate with the other Civs, but it seems so little effort is given to actually making Diplomacy an interesting, fun, strategic aspect of the gameplay.
Being able to barter over anomalies, asteroid belts and planets would be a great feature. Offering promises of "spoils of war" would be a great addition to rally allies to your cause. Forming temporary coalitions as opposed to permanent alliances would be a great addition as well. Perhaps a coalition is formed just to defeat a particular race and then after that is accomplished the individual member races of the coalition go on their separate ways.
However, what I would really love to see is that communication matters in diplomacy. I'd love to see a "mini-game" made out of diplomacy. Perhaps a screen where you are presented with a few different options in terms of how you communicate. You could select different phrases, then select a type of demeanor. This would mean that you really have to be careful about how you communicate with the various races. Perhaps some races respect a more authoritative tone, while others may want you to show more subserviance.
The whole Yes/No/Request aspect of diplomacy is frankly really boring. I think by adding more elements to Diplomacy you can then actually utilize your level of diplomacy research in the game. If you know more about about communication and diplomacy and even through spying know more about a particular race, new options for communication open up for you as you progress through the Diplomacy/Communications tech tree. In the beginning you're left to kind of guess as to how to communicate with the races, how will you phrase your request or your statement and what demeanor will you use? Then as you research Diplomacy and learn more about the species from spying the Diplomacy "mini-game" opens up more options for communicating, gives you more subtle/detailed options for communicating and gives you something like a "chance of success" type read out. Your advisor could say something like "The Drengin are more likely to respond to a humbled request sir."
As you become more powerful as a economy, as a military and as a diplomatic body, new options for communicating open up and the way in which the various races respond to you could change. Perhaps early in the game, when you are weak the Drengin will respond favorably to a humbled approach, but as you gain power and perhaps progress beyond them, then then respond more favorably to a stern approach.
These are just quick ideas off the top of my head, but I think that Diplomacy can be changed/improved to be a legitimate, strategic and enjoyable game mechanic and not just something thrown in to list as a feature on the back of the retail box.